The present invention relates to sialon compositions, especially those that are used as cutting tools for the machining of metallic materials.
Cutting tools with high wear resistance and reliability are critical to improving productivity in the high speed machining of high temperature alloys. High temperature alloys (or superalloys) are generally classified as having poor machinability. Consequently, they are machined at speeds that are typically only 10 to 20 percent of those used for steel when using cemented carbide tools. High temperature alloy machining speeds can be significantly increased, however, by employing silicon nitride, sialon (U.S. Pat. Nos. 4,711,644; 3,991,166; 4,818,635) or silicon carbide whisker reinforced alumina tools (U.S. Pat. Nos. Re. 45,196; 4,789,277 and 4,961,757). Of these, the silicon carbide whisker reinforced alumina tools provide the highest wear resistance but are the most expensive due to the high cost of silicon carbide whiskers, and the fabrication technique required to manufacture these tools, i.e. hot pressing (as opposed to cold pressing and then sintering). Ceramic cutting tools containing sialon typically include beta prime sialon (Si.sub.6-Z Al.sub.Z O.sub.Z N.sub.8-Z) and intergranular phases, or beta prime plus alpha prime sialon [(Si,Al).sub.12 M.sub.X (O,N).sub.16 where X is between 0.1 and 2 and M can be Li, Ca, Mg, Hf, Zr, Ce, Y, Sc or other lanthanides] and intergranular phases. The value of Z for beta prime sialon may be greater than zero but less than 4.2; however, all commercial grades of sialon based (i.e., containing 50 v/o or more sialon) cutting tools have had Z values significantly below 1.0 since it is believed that the toughness of sialons deteriorates as the value of Z increases.
Black ceramics (Al.sub.2 O.sub.3 +TiC) and some white ceramics (Al.sub.2 O.sub.3 and Al.sub.2 O.sub.3 +ZrO.sub.2) also show excellent wear resistance in these applications, but suffer from poor reliability.
Therefore, there is a need for an economical ceramic tool material with a combination of high wear resistance and toughness in high speed machining of high temperature alloys. Similarly, there is also a need for an economical ceramic material with a combination of high wear resistance and toughness in the machining of cast irons, hardened cast irons and hardened steels.